This invention relates in general to land vehicles and more particularly to wheelchairs and side frames therefor. Most particularly, the invention relates to a side frame for a foldable wheelchair.
A conventional wheelchair typically has a pair of side frames that includes front and rear side frame members and upper and lower side frame members arranged to form a generally rectangular frame structure, which is typically oriented in a substantially vertical orientation.
The left and right side frames may be connected together by two or more cross braces to allow the wheelchair to fold such that the left and right side frames move together to create a narrow folded structure. Each cross brace typically has a lower pivot that pivots about or near a lower side frame member of a corresponding one of the left and right side frames. The cross braces cross one another at a cross brace pivot point and pivot with respect to one another about a central longitudinal pivot axis.
In order to control the folding kinematics of the wheelchair, two cross brace linkages are typically employed. The cross brace linkages have upper ends that pivot about longitudinal axes at or near the upper side frame members. Lower ends pivot about longitudinal pivot axes on the cross braces. The cross braces linkages restrict folding motion to a single degree of freedom, making it easy to fold the wheelchair in a single motion. The resulting folding kinematics is such that the left and right wheelchair side frames remain parallel when the wheelchair is unfolded and are generally parallel when the wheelchair is folded.
Left and right seat frame members are typically supported by upper ends of the cross braces so that the seat frame members reside next to and substantially parallel to the upper side frame members when the wheelchair is unfolded. Typically, an upholstery seat sling is secured between the left and right seat frame members to form a seat surface for supporting a wheelchair occupant. In addition, left and right backrest frame members are typically secured to the side frames and flexible backrest upholstery is secured between these backrest frame members. This upholstery forms a backrest surface for supporting the occupant's back.
Generally, the side frames are supported by drive wheels, usually located at the rear of the wheelchair, and by casters, usually located at the front of the wheelchair. To achieve this, the lower frame members typically extend longitudinally from the drive wheels to the casters. Optionally, connecting members may connect the lower frame members to the drive wheels or casters.
Mounting assemblies are commonly employed for mounting the drive wheels and the casters on the side frames. Such assemblies typically incorporate a number of adjustments that allow the wheelchair occupant to customize the wheelchair to his or her anthropometry or driving condition. Some mounting assemblies are adjustable to allow the height of the drive wheels and the casters to be varied. Mounting assemblies provide the ability to adjust the camber of the drive wheels (i.e., the angle of the drive wheels with respect to a vertical plane). For example, a wheelchair with a large camber angle has more responsive turning while a wheelchair with a little or no camber angle has a smaller overall width and thus greater maneuverability in tight confines. Mounting assemblies also provide the ability to adjust the fore and aft positions of the drive wheels with respect to the wheelchair frame. Such adjustment is known as a center-of-gravity adjustment. For example, moving the drive wheels rearward produces a more stable wheelchair that is less likely to tip backwards while moving the drive wheels forward makes the wheelchair easier to balance on the drive wheels. This helps with maneuverability over obstacles, such as curbs, where the wheelchair occupant must lift the casters off the ground in order to traverse the obstacle. Further, mounting assemblies permit the drive wheels to be adjusted laterally with respect to the side frames. Such adjustment allows the wheels to be properly spaced as close as possible to the side frame, while still providing clearance to accommodate optional accessories, such as side guards or armrests. Having the wheels spaced closer to the side frame creates a narrower overall width, allowing the occupant to enter narrow confines.
There are several problems associated with conventional wheelchairs. For example, conventional foldable wheelchairs often have flimsy frames. To help stiffen the frame, thicker walled tubing is often used. Tight tolerance of the folding pivot joints may also be required. However, these requirements add to the weight and the cost of the wheelchair and may make the wheelchair more difficult to fold.
As another example, foldable wheelchairs often have side frames that do not remain parallel throughout the folding motion of the wheelchair. When the wheelchair is nearing the completely unfolded condition, the upper side frame members are further apart than the lower side frame members. This non-parallel arrangement has the effect of causing upper ends of the backrest members to separate wider than the overall width of the unfolded wheelchair. This, in turn, causes the backrest upholstery to overstretch beyond the width of the wheelchair. Providing additional slack in the backrest upholstery to accommodate this condition is undesirable because the backrest upholstery should be taut when the wheelchair is unfolded. This overstretching of the backrest upholstery makes it difficult to fold and unfold the wheelchair, and tends to overstress components of the wheelchair that support the folding pivot axes. As a result, certain components are reinforced and made heavier to deal with this stress, which further adds to the weight and the cost of the wheelchair.
As yet another example, foldable wheelchairs typically have upper and lower side frame members that which contribute to the overall height of the side frames. The height of such wheelchairs is typically so tall that it is difficult to transfer the folded wheelchair over the user's lap when loading and unloading the wheelchair into and out of a car when the user is sitting in the driver's seat.
Still another example of problems associated with wheelchairs is with regard to the cambered drive wheels, which encounter a change in camber axes when the height of the drive wheels or casters is varied. This causes the drive wheels to toe in or toe out. That is to say, the drive wheels become misaligned with respect to the plane of a supporting surface. This misalignment is undesirable because it increases rolling friction. If the drive wheels are raised or the casters are lowered, the drive wheels will toe out. Conversely, if the drive wheels are lowered or the casters are raised, the drive wheels will toe in. This occurs because the axis of the camber is no longer aligned horizontally. To correct this, the mounting assemblies that attach the drive wheels to the side frames must allow the axles of the drive wheels to rotate in order to re-align the camber angle with respect to horizontal.
While some foldable wheelchairs provide height, lateral, camber toe in and toe out and center-of-gravity adjustability of the drive wheels to address the forgoing problems, there is strong demand for a design that offers user-friendly adjustment and is lightweight. There are major challenges in designing foldable wheelchairs with a structure that is sufficiently rigid. Other factors to consider include the type and amount of material used, the number and intricacy of the component parts, and the overall weight of the wheelchair. It would be advantageous to have a wheelchair that required less material or had less intricate members, and is lightweight. It would be advantageous to have a wheelchair that has a shorter overall package size when folded.